The present invention relates generally to optical devices. More particularly, the present invention relates to mode multiplexing optical coupling devices, such as mode multiplexing combiners for use with fiber lasers, fiber pumped solid-state lasers, and optical amplifiers, as well as to optical couplers and splitters based on mode managed optical coupling.
Fused coupler technology, wherein optical fibers are bundled together, heated, and pulled lengthwise, is commonly used to produce couplers, combiners and splitters for optical communication systems, medical devices and other industrial applications. Generally, a combiner is a passive fiber optic coupler in which power from several input fibers is combined into one output fiber. Conversely, a splitter divides light from a single input fiber into two or more output fibers. The coupler represents the general case where inputs from one or more fibers are mixed and distributed among one or more output fibers.
Combiners in particular are seeing new applications for diode pumped lasers, including fiber lasers and solid-state lasers, and diode pumped optical amplifiers. They are used to combine multimode optical pump power from a multiple sources, such as multimode laser diodes, and transfer the combined pump power into the inner cladding of a multiclad fiber or into a multimode fiber. A multiclad fiber typically has a small core (that typically transmits a singlemode or a small number of modes) surrounded by an inner cladding layer of lower refractive index and significantly larger cross-section that transmits the multimode pump power. An outer cladding of even lower refractive index causes the multimode pump power to be confined in the inner cladding by total internal reflection. The multiclad fiber is used to combine a singlemode (or multimode mode) signal in the core, along with multimode pump power in the inner cladding, to a separate device which may be used for amplification. These mode multiplexing combiners are typically used with cladding-pumped fibers. Cladding-pumped fibers are a special case of multiclad fiber where the multimode light propagates within the core and inner cladding interacting with special dopants (such as rare-earth elements like Er) in the core that absorb the pump photons and radiate photons at a different wavelength. Under suitable conditions, the special dopants in the core cause stimulated or spontaneous emission at the different wavelength and can operate in the form of a fiber laser or optical amplifier. Multiclad fibers containing special dopants for the purpose of lasing or amplification are known as cladding-pumped fibers.
For any coupler, splitter, or combiner, it is desirable to maximize the throughput of optical power from any input fiber, through the device, and through any output fiber. For convenience, the case of a combiner is further described, recognizing that the same principles apply equally to splitters and couplers. The throughput depends on efficiently transferring the total brightness from all the input fibers into a single output fiber having sufficient capacity to carry the combined brightness. This transfer can be analysed using modal analysis, ray-tracing methods, or by simple matching of input and output brightness. Conservation of brightness is based on the LaGrange Invariant of an optical system and is typically characterized by the quantity etendue, which is the product of the area of illumination times the extended solid angle. For a fixed level of optical power, increased brightness implies a decrease in etendue. For a step index multimode optical fiber, the etendue can be approximated by E=xcfx802/4 NA2 D2. If such a step index fiber is tapered, its etendue remains constant while the effective numeric aperture (NA) increases as the diameter (D) decreases. In this analysis, the NA refers to the maximum angle of light entering or exiting the optical fiber according to NA=sin(acceptance angle).
In order to efficiently transfer power between two optical elements (in this case from an input fiber bundle into an output fiber), two requirements must be satisfied. First, the etendue on the input side should be less than or equal to the etendue on the output side, otherwise the coupling efficiency will be limited by Eout/Ein. Second, the areas must be matched at the junction.
Prior art combiners, such as that described in U.S. Pat. No. 5,864,644 to DiGiovanni et al., rely on the tapering process to eliminate interstitial voids between input fibers, and to develop a suitable circular cross-section in the input fiber bundle. However, it is not possible to solely rely on the tapering process to achieve the requirements for low loss combiners, especially when there is a range in the number of input ports, or if the output fiber is non-circular.
It is, therefore, desirable to provide an improved optical coupling device that substantially eliminates interstitial spacing between input fibers, while providing a good cross-sectional match between the input fiber bundle and the output fiber independent of the number of fibers bundled together.
It is an object of the present invention to obviate or mitigate at least one disadvantage of previous optical coupling devices, such as combiners and splitters. It is a particular object of the present invention to provide an improved optical coupling device that has reduced insertion loss in a wide range of combinations of fiber types, sizes, shapes, and number of ports.
In a first aspect, the present invention provides a tapered optical fiber bundle. The tapered optical fiber bundle consists of a plurality of optical fibers formed into a fiber bundle with a minimized encircling radius. The bundle is adiabatically tapered, and heavily-fused into an induced compact shape with fibers minimally deformed and no interstitial space between the optical fibers. The optical fibers can be multimode, singlemode, multiclad, or cladding-pumped, and one or more of the ports may be terminated. Terminated ports are fibers that are included in the bundle only to form part of the geometric or optical structure, but are subsequently cut-off outside the fused region using a technique to minimize reflection from the cut endface.
In a further aspect, the present invention provides an optical fiber device, such as an optical combiner, an optical splitter, an optical coupler, for use in an optical system such as a cladding-pumped fiber laser, fiber pumped solid-state laser, or a cladding-pumped optical amplifier. The optical fiber device consists of a tapered fiber bundle coupled to a multimode, multiclad, or cladding-pumped optical fiber, a second tapered fiber bundle, or a bulk optical device (such as a solid-state laser element). The coupling process maximizes the transfer of power (and signal) from the inputs to the outputs by virtue of the optimized brightness contained in the input bundle.
In yet another aspect, the present invention provides a method of manufacturing the tapered fiber bundle. First, a plurality of optical fibers, decoated to remove any polymer or metallic coating layers in the region where they will be heated, are positioned in a predetermined configuration that will result forming a minimized encircling radius. The positioned fibers are then twisted and bundled under controlled tension to result in a fiber bundle with minimized encircling radius. An adhesive can be used at both sides of the bundle to secure the positioning. The bundle is then heated and pulled to heavily fuse the fibers, while adiabatically tapering the bundle, into an induced shape with no interstitial space between minimally deformed fibers. If desired, glass cladding can be fully or partially removed prior to fusing the bundle. Also one or more singlemode (or multimode) fibers can be incorporated into the bundle at appropriate locations such that their position in the resulting tapered bundle corresponds to similar singlemode (or multimode) cores in an output bundle or multiclad fiber or cladding-pumped fiber.
To form an optical fiber device according to the present invention, the fused and tapered bundle is cleaved at the tapered region. The cleaved bundle endface is optionally reshaped by fusion splicing it to an output fiber of appropriate shape, then recleaving again. The splicing and re-cleaving can be repeated, if desired, until surface tension during fusion splicing causes the cleaved end to approach the desired cross-sectional geometry. The reshaped cleaved end is then coupled to a suitable optical element, such as a single optical fiber, a second tapered fiber bundle, or a bulk optical device (such as a solid-state laser element). The method of the present invention can include pre-tapering of the output fiber, post-tapering of the junction between the tapered fiber bundle and an output fiber or bundle, and re-coating the junction with a coating material, such as polymer or metallic material.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.